The present invention relates generally to an improved process for producing alkyl glyoxylates and, more particularly, to a process for separating an alkyl glyoxylate from the corresponding alkyl glycolate used to produce it and various other reaction constituents.
One method for producing alkyl glyoxylates is by the oxidative dehydrogenation of alkyl glycolates in the presence of a silver catalyst. For each mole of glyoxylate, one mole of water is produced. Additional water can be provided by side reactions, along with lesser amounts of the alcohol corresponding to the alkyl moiety of the glyoxylate and various other minor by-products. Product isolation is complicated by liquid phase reactions between the alkyl glyoxylate and unconverted glycolate or co-products (most notably hydroxyl species such as water and the alcohol corresponding to the alkyl moiety of the alkyl glycolate). These reactions are of two general types, viz., those which reverse when heated and those which are thermally irreversible. Examples of these two reactions are: ##STR1##
The thermally reversible reactions produce families of nonvolatile polymeric hemiacetals that can be decomposed to the monomeric species by the addition of heat. The irreversible reactions represent a permanent loss of product. Therefore, isolation of the glyoxylate by simple distillation is impossible, although more complex schemes can be successfully employed.
A typical reaction mass contains more total alkyl glycolate, water and the alkanol corresponding to the alkyl glycolate, on a molar basis, than alkyl glyoxylate. Because there are more impurities than alkyl glyoxylate, the bulk of the glyoxylate will be involved in the thermally reversible and irreversible reactions. If the amount of impurities is reduced, the interactions between the alkyl glyoxylate and the other reaction constituents will be reduced, and the alkyl glyoxylate can be more easily recovered. If no water or alcohol were present, there would be no thermally reversible or irreversible reactions with water or alcohol, although the reactions between the alkyl glycolate and alkyl glyoxylate would still occur. To maximize the production of free alkyl glyoxylate, methods have been sought to reduce the amount of water and alcohol, as well as the amount of alkyl glycolate. In the past, decreasing the content of the water and the alcohol corresponding to the alkyl glycolate has been achieved by distillation or by adding a species that reacts with or azeotropes the water and alcohol. The amount of alkyl glycolate remaining in the reaction mass can be minimized by achieving higher reactor conversions.
U.S. Pat. No. 4,502,923 discloses a method to isolate alkyl glyoxylate from its impurities in a series of distillations. A low boiling azeotroping agent is introduced to cause water and alcohol to distill overhead. The water and alcohol content is then lowered by a vacuum distillation. The residue from the first column is distilled in a second column to decrease the ratio of alkyl glycolate to alkyl glyoxylate. The residue of the second column is then distilled at a higher pressure in a third column to recover the high purity alkyl glyoxylate as a side draw. To produce high purity alkyl glyoxylate in the prior art process, it is necessary to have a large recycle stream on the order of 11 lbs. of recycle throughout the three distillation columns per pound of product takeoff. Consequently, the capital investment in plant equipment is high, while the production of product is low.
Australian Patent No. 30007 -84 discloses a method for isolation of the glyoxylic ester from materials co-formed during an oxidative dehydrogenation of an alkyl glycolate. Immediately after leaving the reactor, the gaseous reaction mixture is quenched with a low-boiling entrainer, such as a hydrocarbon, having a lower boiling point than the glyoxylic ester. The entrainer is used to azeotropically distill water and alcohol. The reaction mixture, along with the entrainer, is passed to a fractionating column, where the water azeotrope and other low boilers are taken overhead while the glyoxylic ester is taken off as bottoms. The method has the disadvantage that some of the water and alcohol in the reaction mixture forms high-boiling hemiacetals and hydrates before they can be azeotropically distilled, requiring the use of additional steps to produce high purity alkyl glyoxylate.
It is also known that high purity alkyl glyoxylate can be isolated from a mixture containing alkyl glyoxylate, water, and alcohol in combined form by distillation from phosphorous pentoxide [W. Oroshnik and P. E. Spoerri, J. Amer. Chem. Soc. 1941, 63,3338 and J. M. Hook, Synthetic Communications, 14(1), 83-87 (1984)]. Product losses are high and a highly corrosive waste stream is formed, which poses difficult disposal problems. In addition, the reaction with P.sub.2 O.sub.5 is extremely exothermic. Consequently, this technique is suitable only for small-scale operations.
A simple and cost-effective method to isolate alkyl glyoxylate is desired. If an economical method were developed which minimized the contact between the alkyl glyoxylate and the various hydroxyl groups, the thermally reversible and irreversible reactions could be inhibited and product isolation improved.